Metal ions such as copper and zinc are indispensable cofactors in the chemistry of living systems. For example, copper, with two oxidation states Cu(I) and Cu(II), is critical to cellular redox reactions. In plants, copper-binding proteins utilize this metal as a co-factor in diverse biochemical pathways, e.g., photosynthesis, respiration, oxidative stress response, and ethylene signaling. Yet, while copper and other metal ions are pivotal to cellular biochemistry, they lead to the production of highly destructive hydroxyl radicals when present in excessive concentrations. In fact, no free copper ions are found in cells, as they are chelated either by low-molecular weight compounds (e.g., nicotinamine) or by copper-binding proteins (e.g., metallothionein).
To date, several studies have identified short sequence metal-binding motifs. These can be exploited, vis-à-vis the chemically reactive properties of metal ions, to develop enzymes having improved or new catalytic properties. Such novel enzymes can be used in a variety of industrial or biomedical applications. Polypeptides containing metal binding motifs can also be used for chelating metal ions to reduce their free concentrations in metal-contaminated substrates, e.g., industrial waste, soil, or water, in an economically and ecologically sound way. For this purpose, metal binding polypeptides can be added directly to a contaminated substrate, or alternatively, transgenic cells that express metal-binding polypeptides can be used for bioremediation.